This invention relates to a shadow mask assembly adapted to be incorporated into a color cathode ray tube and more particularly to a flat shadow mask assembly.
Generally, a shadow mask assembly comprises a shadow mask, a reinforcing frame to fit and support the shadow mask and leaf springs secured to the frame for supporting the assembly.
More particularly, as shown in FIG. 1, a prior art cathode ray tube for use in a color television receiving set comprises an envelope having a face panel 1 with its inner surface coated in the conventional manner with a film of phosphor in the form of a plurality of phosphor dots, and a shadow mask assembly 2, shown in exploded perspective. The shadow mask assembly includes a rectangular shallow dish-shaped shadow mask 3 provided with a large number of small perforations, for example from 100,000 to 400,000, a reinforcing frame 4 made of a metal strip having an L-shaped cross-section, and an electron shield 5 secured to the reinforcing frame 4, which is fitted in the opening of shadow mask 3 to support the same.
A reinforcing frame, such as frame 4, is generally prepared by drawing a bending line along the length of a flat metal strip cut to a predetermined length, bending the strip along the bending line into an L-shaped cross-section, bending the strip into a rectangular configuration and welding together the opposite ends of the strip. Since the strip is bent within a common plane, each side of the completed frame tends to bulge outwardly due to the resiliency of the strip. Thus, the completed reinforcing frame is somewhat elliptical, resulting in strain or deformation of the shadow mask held thereby. Moreover, since the peripheral edge of the electron shield 5 secured to the reinforcing frame 4 extends outwardly beyond the periphery of the reinforcing frame, it is impossible to project the leaf springs 41 forwardly, as viewed in FIG. 1, beyond the periphery of the electron shield, which leaf springs 41 are secured to the reinforcing frame for the purpose of mounting the shadow mask assembly on the face plate 1. For this reasons, when mounting the shadow mask assembly upon the face plate, it is difficult to mount the assembly by grasping the leaf springs.
One approach to this problem is the use of a curved reinforcing frame, as shown in FIG. 2. The four sides of the reinforcing frame 6 are bent to conform to the curved front surface of the shadow mask 3. To prepare such a curved frame, since three dimensional bending operations are required, the tendency of the reinforcing frame to spring back is small so that the frame does not impart any strain or deformation to the shadow mask fitted therein. With this construction, since it is possible to project the free ends of leaf springs 61 secured to the periphery of the frame 6 beyond the edge of the electron shield, the shadow mask assembly can be mounted upon the face panel by grasping these projected ends. However, with this construction, it is also necessary to form the electron shield to be mounted on the reinforcing frame so that it has curved sides. Further, it is necessary to form notches on the periphery of the electron shield to receive the projecting ends of the leaf springs. Such notches permit electrons to pass thus off-setting the purpose of the electron shield.